With steady advances in the processing speed of computers, voice recognition technology for recognizing a user's utterances with a computer has come to be widely adopted. Moreover, voice interaction technology allowing acoustic interaction between computer and user through voice recognition technology and voice synthesis technology is garnering attention in many fields as one possible interface for making use of a computer without using a keyboard or a mouse. These technologies are used in a variety of fields, such as voice portals with which relevant information such as stock quotes or news can be obtained through a telephone line, car navigation systems that can be operated by voice while driving, mobile phones that can be operated by voice, or ticket reservation systems with which flight reservations can be placed by voice.
In conventional voice interaction technology, it is usually assumed that the user interacts by following one scenario of the interaction system. Moreover, in the case of voice portals providing a plurality of interactive services, the service structure is branched like a tree, and is not laid out for using a plurality of interactive services in parallel. Such interactive services are relatively effective for pursuing a single task, such as providing specific information or realizing a specific task.
However, the way that voice interaction services are used by the user is not necessarily limited to pursuing a single task, and it is also conceivable that a plurality of tasks are executed in parallel. For example, if the user finds an entry for a business trip while using an interactive service for schedule management, the user might want to confirm the weather at the locale of the business trip, and then return to the schedule management. Thus, it is desirable that the user can use interactive services in parallel, that is, to switch at any time between a plurality of interactive services that are running concurrently, and to return to a previously performed task.
Japanese Patent Application No. 2003-81136 proposes a context-sensitive complex voice interactive service, with which a plurality of voice interactive services are operated in parallel, and the user can switch the interactive service at any time, or the interactive service spontaneously addresses the user. In this system, in order for the user to freely switch between a plurality of voice interactive systems, the recognition lexicon needs to include not only the recognition terms accepted by the interactive service that is directly interacting with the user at a given time, but also the recognition terms that can be recognized by interactive services that are on stand-by in the background.